HandBrake

I am loading an XDCAM HD 4:2:2 clip with 8 channel audio into Handbrake

channel 1 has the sound of the person on the left of the screen
channel 2 has the sound of the person on the right of the screen
channels 3 - 8 are unused

in Handbrake, it only loads the default channel1 and if I export, only the sound of the person on the left appears
if I add another track and set it to source channel2, and then export, all you hear is the sound of the person on the right, it seems that whatever is the bottom track will be heard and nothing that is above will be heard

how can I set it so you hear both sounds in the final mp4?

see version info and settings in attached video
http://www.videoproductions.com.au/HB/HB-issue.mp4

You'll need to mix the two mono tracks into a single stereo track before transcoding with HandBrake.

ok, will do that if Handbrake can't do the mixdown